The present invention relates to cathode ray tubes and, more particularly, to a cathode ray tube with improved contrast.
A cathode ray tube typically includes a glass-made outer envelop which is designed to consist essentially of a panel section for visually displaying images, a neck portion housing therein an electron gun assembly, and a funnel section for coupling the panel section and the neck portion together.
An electron beam that emitted from an electron gun impinges on a layer of fluorescent or phosphor material that formed on the inner surface of a face plate, thereby permitting light emission of the phosphor material. The face plate has its part with picture elements or xe2x80x9cpixelsxe2x80x9d formed therein, which is for use as a display screen. A color cathode ray tube has been provided which has its phosphor layer that is reduced in pitch in order to display high-resolution images. The quest for higher resolution of on-screen images results in the improvement in display image contrast required.
It is also noted that color cathode ray tubes of the flat panel type with a front panel face made substantially flat have been widely employed as picture tubes of television receivers and/or personal computer monitor units. Screen flattening makes it possible to improve on-screen image viewabilities.
Since the glass envelop of a cathode ray tube is evacuated to a high degree of vacuum in its interior space, plate thicknesses at respective portions of the glass envelop are set at specific values for enabling them to withstand atmospheric or barometric pressures. Especially, the face plate of a flat-panel type cathode ray tube is such that the plate thickness of a peripheral portion is greater in value than that at a central portion.
Due to this, the brightness or luminance of an image being displayed on the face plate decreases at peripheral portions, as compared to that at the central portion of the face plate. Furthermore, the weight of phosphor becomes smaller at the screen periphery than at the screen center, resulting in a further decrease in luminance. To preclude such luminance reduction at the periphery, certain panels with transmittance of more than 70% are usually employed.
However, the use of such high-transmittance panels can result in a decrease in contrast of images.
One known technique for improving the image contrast is to fabricate a colored film on the front face of a panel for appropriate adjustment of spectral transmittance. It is well known among those skilled in the art that the colored film is formed by sol-gel methods. For example, deposit on the panel""s front face a mixture liquid of metal alkoxide and alcohol along with water and coloring pigment, and thereafter perform baking or sintering it to thereby form the colored film required.
Since the pigment readily exhibits flocculation in a metal alkoxide liquid, it has been difficult to retain dispersion of pigment in the metal alkoxide liquid for an increased length of time period. If flocculated pigment resides on the panel face then light rays can scatter or disperse due to such flocculated pigment, resulting in loss of optical transparency. Further, the presence of the flocculated pigment would result in lack of clearness or crispness of display images, leading to blur thereof.
On the other hand, addition of an increased amount of dispersing agent into the mixture liquid for suppression of pigment flocculation would disadvantageously result in a decrease in physical strength of the colored film.
When forming a colored film by sol-gel methods, a specific mixture liquid containing metal alkoxide and coloring pigment along with fine or micro-particles of metal oxide and water plus alcohol is deposited on the front face of a panel, thus forming the intended colored film through a baking or sintering process. This colored film contains therein colloidal metal for facilitating dispersion of the pigment. Use of the colored film containing the colloidal metal makes it possible to obtain the colored film of less flocculation.